This invention relates to electric discharge machining apparatuses, and more particularly to an electric discharge machining apparatus for forming a through-hole in a workpiece, in which, when the electrode penetrates the workpiece, the machining operation is automatically ended.
In an electric discharge machining operation, an electrode confronts the workpiece to be machined with a gap therebetween, and an electric discharge is caused in the gap while a machining solution is supplied to the gap, to machine the workpiece as required. If an electric discharge machining apparatus is used to bore a hole in a workpiece, a trimming die having a desired configuration can be formed with high accuracy. Therefore, this method is suitable for forming various metal or other molds. However, conventional methods of forming a hole in a workpiece using electric discharge machining apparatuses are disadvantageous in the following points: Firstly, the time instant when the end of the electrode penetrates the workpiece cannot be automatically detected. Accordingly, although the hole has been formed in the workpiece, the electrode is still supplied with current to continue the electric discharge machining operation. As a result, the nominal machining time is increased by as much, and the side of the trimming die is excessively machined i.e., machining accuracy is considerably lowered. Furthermore, heretofore, in order to detect the penetration of the electrode, the amount of feed of the electrode has been merely estimated, or the operator was required to watch the movement of the electrode, during the machining operation.
FIG. 1 illustrates a conventional electric discharge machining apparatus. An electrode 10 and a workpiece 12 to be machined confront each other, with a gap therebetween, in a machining solution contained in a machining tank 14. A gap voltage is applied across the gap by a power source unit 16. The gap voltage is adjusted according to a detection value of a gap voltage detecting unit 18, so that electric discharge machining is carried out under the condition that the gap is maintained constant. The machining solution is supplied from a machining solution circulating unit 20 into the machining tank 14 by a supply pump 22. In this operation, the pressure of the machining solution is controlled according to a pressure value which is detected by a machining solution pressure gauge 24. In the conventional electric discharge machining apparatus thus constructed, while the gap between the electrode 10 and the workpiece 12 is maintained unchanged, the electrode 10 is moved in the machining direction (downwardly in FIG. 1), so that a recess or a through-hole is formed in the workpiece 12 by the electric discharge which occurs in the gap.
With the conventional apparatus described above, after the electrode 10 has penetrated the workpiece 12, the electrode 10 is further moved in the machining direction (or downwardly in FIG. 1). Accordingly, it is necessary for the operator to watch the position of the electrode 10 relative to the workpiece 12 at all times; that is, it is necessary for him to control the speed of movement of the electrode 10 with a dial gauge (not shown) or the like, so that the electrode 10 is stopped simultaneously when the workpiece has been machined. This operation is undoubtedly troublesome. On the other hand, in the case where the amount of consumption of the electrode 10 (or the amount of movement of the electrode 10) is known in advance, heretofore the following method has been employed: A dial gauge operated in association with the movement of the electrode 10 is used in combination with a limit switch, so that when the electrode 10 has moved a predetermined amount set in advance, the machining operation is automatically ended.
As described above, with the conventional electric discharge machining apparatus, the operator is required to watch the position of the electrode 10 relative to the workpiece 12 at all times, so as to detect when the electrode 10 penetrates the workpiece 12, to thereby end the machining operation. Thus, the machining operation is tedious and troublesome, and it is difficult to maintain the amount of feed of the electrode 10 constant. Furthermore, the conventional electric discharge machining apparatus is disadvantageous in that, in the case where a through-hole is formed in the workpiece with a thin electrode, the electrode is vibrated depending on the amount of feed of the electrode after the electrode has penetrated the workpiece; that is, it is not moved in a straight line after penetrating the workpiece, as a result of which the configuration of the hole thus formed is adversely affected.
In the case where the amount of consumption (or the amount of movement) of the electrode 10 is known in advance as described above, the machining operation can be ended automatically to some extent by using a dial gauge operating in association with the movement of the electrode and a limit switch (not shown). However, under the electrical conditions in an ordinary hole forming operation, the electrode 10 is greatly consumed, and therefore it is difficult to determine the amount of consumption (or movement) of the electrode 10 in advance.
As described above, in order to detect when the electrode has penetrated the workpiece to thereby end the machining operation, the operator must carry out troublesome work, which lowers work efficiency and makes it difficult to provide an automatic electric discharge machine.